Hydrogen gas is a very desirable fuel because it can be reacted with oxygen in hydrogen-consuming devices, such as a fuel cell, combustion engine or gas turbine, to produce energy and water. The use of hydrogen gas can ameliorate environmental pollution; lessen the world's dependency on fossil fuels or petroleum; ease fears of depleted energy sources.
Hydrogen (H2) is expected to be the principal energy source or “the fuel of the future” and can be used in H2 engine cars and future power devices, as described in “Basic Research Needs for the Hydrogen Economy,” Report of the Basic Energy Sciences Workshop on Hydrogen Production, Storage, and Use, May 13-15, 2003, http://www.sc.doe.gov/bes/besacBasicResearch_Needs_To_Assure_A_Secure_Energy_Future, second printing, February 2004.
Some of the future power devices include solid oxide fuel cells as reported by H. T. Wang et al. in “Hydrogen-selective sensing at room temperature with ZnO nanorods,” Appl. Phys. Lett. 86 (2005) 243503-243505 and L. C. Tien et al. in “Hydrogen sensing at room temperature with Pt-coated ZnO thin films and nanorods,” Appl. Phys. Lett. 87 (2005) 222106-222108. However, H2 is a hazardous, odorless, and highly inflammable gas and it is very important to detect its leakage. A reliable and inexpensive sensor that can take advantage of nanoscale technology to detect hydrogen leaks is the focus of many research groups including H. T. Wang et al., Appl. Phys. Lett. 86, supra. L. C. Tien et al. Appl. Phys. Lett 87, supra, B. S. Kang, in “Hydrogen and ozone gas sensing using multiple ZnO nanorods,” Appl. Phys. A 80 (2005) 1029-1032, J. X. Wang, et al. in “Hydrothermally grown oriented ZnO nanorod arrays for gas sensing applications,” Nanotechnology 17 (19) (2006) 4995-4998, F. Favier, et al. in “Hydrogen sensors and electrodeposited palladium mesowire arrays, Science 293 (2001) 2227-2231, and G. C. Yi et al. in “ZnO nanorods: synthesis, characterization and applications, Semicond. Sci. Technol. 20 (2005) S22-S34.
Common sensors propose use of an indirect approach (e.g. Raman spectroscopy, etc.) or requiring complicated components to detect the presence of H2. Many ideas have been proposed such as use of different metal wires, semiconductor oxides nanoarchitectures, and the like. Gas sensors based on ZnO nanorods, SnO2 nanowires, In2O3 nanowires, etc. showed excellent response and recovery characteristics, as reported by L. Liao et al. in “Size dependence of gas sensitivity of ZnO nanorods,” J. Phys. Chem. C 111 (5) (2007) 1900-1903 and can potentially overcome obstacles of other types of sensors, such as sensitivity, selectivity, and the like.
Among different nanomaterials, nano-ZnO is one of the most promising multifunctional materials for gas sensors, especially for H2 sensing. ZnO nanorods also have the advantages of large surface area, radiation hardness, as reported by D. C. Look in “Recent advances in ZnO materials and devices, Mater. Sci. Eng. B 80 (2001) 383-387, thermal and mechanical stability, as discussed by Ü. Özgür et al. in “Comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98 (2005) 041301. Properties of the nano-ZnO materials depend on the microstructures including morphology, crystal size, orientation, aspect ratio, and crystalline density, according to Z. R. Tian et al. in “Complex and oriented ZnO nanostructure, Nat. Mater. 2 (12) (2003) 821-826.
Sensing properties of nano-ZnO are directly related to its preparation history, particle size, surface to volume ratio, morphology, and operating temperature. The signal consists of conductivity changes due to gas adsorption on the surface and permits real-time detection.
Recently, the branching growth phenomena such as nanojunction arrays have attracted great interest for achieving high degree of superior functionality via direct self-assembly. Thus, multiple ZnO nanorods, as discussed by B. S. Kang et at in Appl. Phys. A 80, supra and single ZnO two-dimensional branched nanorods have attracted considerable attention due to their unique properties that strongly depend on their size, morphologies reported by J. Y. Chen, et al. in Angew. Chem. Int. Ed 44 (2005) 2589 and configurations reported by O. Lupan, et al. in “Synthesis and characterizations of ZnO nanorods arrays and mesoporous films for device applications, in Proceedings of NSTI Nanotechnology Conference and Trade Show, Santa Clara, Calif., USA, 20-24 May 2007, V. 4, 457-460; and O. Lupan et al. in “In-situ lift-out fabrication and characterizations of ZnO branched nanorod-based sensors,” Proceedings of NSTI Nanotechnology Conference and Trade Show, Santa Clara, Calif., USA, 20-24 May 2007, and their possible use as low-dimensional building blocks or functional units in H2 nanosensors and nanodevices, according to G. C. Yi in Semicond. Sci. Technol. 20, supra and Z. P. Sun et al. in “Rapid synthesis of ZnO nanorods by one-step, room-temperature, solid-state reaction and their gas-sensing properties,” Nanotechnology 17 (2006) 2266-2270.
Different H2 sensors have been demonstrated. Wang et al. in Appl. Phys. Lett. 86, supra used multiple ZnO nanorods with Pd and achieved 4.2% relative response (ΔR/R) at 500 ppm H2 in N2 after 10 minutes exposure.
According to experimental results presented by Wang et al. Nanotechnology 17, supra the H2 sensitivity of nanowires has the highest sensitivity (S˜8) at 250° C. Tien et al. in “Room-temperature hydrogen-selective sensing using single Pt-coated ZnO nanowires at microwatt power levels,” Electrochem. Solid-State Lett. 8 (9) (2005) G230-G232 used single Pt-coated ZnO nanowires and achieved a relative response of ˜20% at 500 ppm H2 in N2 after 10 minute exposure. At the same time realized sensitivity (Smax=Rair/RH2 is between 1 and 2) at 100 ppm H2 with ZnO nanorods is several times higher than the sensitivity of ZnO films at 300-400° C. obtained by Y. Min et al. in “Gas response of reactively sputtered ZnO films on Si-based micro-array,” Sens. Actuators B: Chem. 93 (1-3) (2003) 435-441.
Tien et al. Appl. Phys. Lett. 87, supra demonstrated a current response of a factor ˜3 times larger for Pt-coated multiple ZnO nanorods versus ZnO thin films upon exposure to 500 ppm H2 in N2 at room temperature. Tien et al. also found that the ZnO multiple nanorods sensors showed a faster response and a slower recovery in air after H2 exposure than ZnO films.
Single nanowires of different metal oxides are discussed By C. S. Rout, et al in “Room temperature hydrogen and hydrocarbon sensors based on single nanowires of metal oxides,” J. Phys. D: Appl. Phys. 40 (2007) 2777-2782 and metal catalyst coatings (Pt, Pd, Au, Ag, Ti and Ni) on multiple ZnO nanorods which are easy to fabricate and possess enhanced sensing properties are reported by H. T. Wang et al in “Detection of hydrogen at room temperature with catalyst-coated multiple ZnO nanorods,” Appl. Phys. A: Mater. Sci. Process. 81 (6) (2005) 1117-1119.
Although ZnO nanorods sensors had a high response, high selectivity, and short response time to low concentrations of gas, at the current stage, it is still difficult to obtain single nanowire and to fabricate this kind of device in large quantities according to C. Wang in “Detection of H2S down to ppb levels at room temperature using sensors based on ZnO nanorods, Sens. Actuators B 113 (1) (2006) 320-323.
In the sensor development field some of the considerations are increased complexity, lengthy sample preparation and device fabrication, time consuming analysis and selectivity. Thus, for wide applications of ZnO nano/microrods in sensors, an inexpensive and environmentally benign self-assembly synthesis process is required in order to synthesize transferable nanorods which can be easily handled with modern equipment.
The latest research efforts are directed towards obtaining alternative, lightweight and flexible nanodevices as pointed out by E. Galoppini et al. in “Fast electron transport in metal organic vapor deposition grown dye-sensitized ZnO nanorod solar cells, “J. Phys. Chem. B 110 (2006) 16159-16161 and A. Du Pasquier, et al. in “Dye sensitized solar cells using well-aligned zinc oxide nanotip arrays,” Appl. Phys. Lett 89 (2006), 253513.
The present inventors have published the following journal article, Oleg Lupan et al. in “Fabrication of ZnO nanorod-based hydrogen gas nanosensor,” in Microelectronics Journal 38 (2007) pages 1211-1216 which appeared after the filing of U.S. provisional patent application Ser. No. 61/053,846, filed May 16, 2008; both the publication and the U.S. provisional patent application Ser. No. 61/053,846 are incorporated herein by reference.
The present invention is focused on the fabrication of single and branched (tripod) ZnO nanorods as the material template for designing a H2 sensor. The present invention also provides a synthesis route for self-assembled ZnO nanorods easily transferable to other substrates and in-situ lift-out techniques using focused ion beam (FIB) system to fabricate individual nanosensor with at least one electrode, for a single ZnO nanorod, or three electrodes for a branched, tripod shaped ZnO nanorod, to detect H2 at room temperature and provide a sensor that is quite selective in sensing H2 in an environment with other gases.